1. Field of the Invention
The present invention is related generally to telecommunication and, more specifically, to a technique for detecting and compensating for time of arrival errors in a telecommunications system.
2. Description of the Related Art
Emergency services are often requested using telephone numbers, such as “911.” If the caller is in a fixed location, such as a residence, computer systems track the telephone number of an incoming telephone call using automatic number identification (ANI) and quickly determine the address from which the call originated. Thus, it is a relatively simple task to determine the location from which emergency services are requested.
The location of a user requesting emergency service via mobile communications, such as cellular telephones, personal communication systems (PCS) devices and the like, is not as easily determined. Radio triangulation techniques have long been used to determine the location of a mobile unit. However, such radio triangulation techniques are known to be inherently inaccurate. Errors on the order of thousands of meters are not uncommon. However, such errors are unacceptable for the delivery of emergency services.
The Federal Communications Commission (FCC) has ordered changes in communication technology that will permit greater accuracy in location determination. In the case of mobile communications, the FCC has generated a rule that requires infrastructure based location systems to have an accuracy of 150 meters 67% of the time (and an accuracy of 300 meters 95% of the time). For systems that require modified handsets, the FCC has decreed that such systems must determine location within 50 meters 67% of the time (and 150 meters 95% of the time).
Existing position location technologies based on global positioning system (GPS) use a network of satellites in the sky that transmit signals at a known time. A GPS receiver on the ground measures the time of arrival of the signals from each satellite it can detect. The time of arrival, along with the exact location of the satellites and the exact time the signal was transmitted from each satellite is used to triangulate the position of the GPS receiver. A typical GPS receiver requires four satellites to make a triangulation, and the performance of the resulting calculation increases as the number of satellites that can be detected increases.
In an alternative to GPS, an existing network of cellular base stations can be treated as a network of satellites for purposes of position location. Similar to GPS technology, the exact location of each base station, the exact time at which the base station is transmitting a signal, and the time of arrival of the base station signal at a mobile station can be used to triangulate the position of the mobile station. This technique is described by some service providers as advanced forward link trilateration (AFLT). A significant problem faced by the mobile station is to measure the time of arrival with the signals that are received from each base station. Different wireless technologies may take different approaches to time of arrival measurements. Code division multiple access (CDMA) is one such technology. CDMA modulation is one of several techniques that allow a large number of system users to share a communication system. It is possible to utilize conventional CDMA modulation techniques as part of an AFLT system.
Radio location systems use time of arrival (TOA) signals coming from different transmitters of known positions to triangulate and estimate the mobile unit location. However, TOA signals are often distorted or erroneous due to multiple transmission paths or the unavailability of a line of sight between the network antenna and the mobile station. FIG. 1 illustrates an example of multiple transmission paths that may be experienced by a mobile phone in a vehicle 10. In the example illustrated in FIG. 1, the mobile unit 10 is receiving signals from transmitters 12 and 14 mounted atop towers. In the example of FIG. 1, the mobile unit 10 receives a signal directly from the transmitters 12 and 14 but also receives signals from the transmitter 14 that have reflected off nearby buildings. Thus, the mobile unit 10 receives a number of signals from the transmitter 14. In the example illustrated in FIG. 1, the mobile unit 10 is not within the line of sight (LOS) of the transmitter 16. That is, buildings or other structures block the direct line of sight between the mobile unit 10 and the transmitter 16. However, the mobile unit 10 still detects signals from the transmitter 16 that are reflected off buildings or other structures or are defracted around edges of buildings or other structures. In addition, the mobile unit 10 receives signals from a transmitter 16 mounted atop a building and may also receive signals from a global positioning system (GPS) satellite 18 in orbit about the earth. As a result, the mobile unit 10 receives multiple signals from the transmitter 16, none of which are direct LOS signals. Signals from the GPS satellite 18 may also comprise both LOS signals and reflected signals.
As a result of such multipath signals, the time of arrival measurements by the mobile unit are subject to error. Such errors can be significant in the presence of multipath signals, thus making it difficult or impossible to achieve the FCC directives with regard to location accuracy. Therefore, it can be appreciated that there is a significant need for a system and method to improve TOA measurements for mobile location systems. The present invention provides this and other advantages that will be apparent from the following detailed description and accompanying figures.